Developing alternative and renewable energy sources is of interest due to environmental concerns associated with using fossil fuels, as well as the need for energy security. Several different types of renewable energy sources have been developed, including wind power, hydropower, solar power, biomass fuels, geothermal energy, and nuclear power. However, some of these energy sources, such as wind and solar power, are not constant and/or readily transported. Thus, it is often necessary or convenient to transform the energy source into electrical energy for transport and then to store it, e.g., as heat with thermal storage or as chemical energy in batteries or capacitors.
In addition, much effort is being made in the automobile industry to try to replace the combustion engine with clean technology to reduce pollution. Possible replacements include two different types of electrochemical devices, i.e., fuel cells and batteries. Fuel cells can convert chemical energy from an environmentally friendly fuel (e.g., hydrogen) into electricity through electrochemical reaction with oxygen. However, for this approach to be feasible, new methods of fuel production, as well as systems for fuel storage and nation-wide transport would need to be developed and/or built.
Batteries and super capacitors transfer and store electrical energy as chemical energy and electrical field energy. In particular, batteries store electrical energy as chemical oxidation-reduction energy and convert this chemical energy back into electrical energy when in use. Chemical energy can be stored in the active materials of a battery negative electrode. When discharging, electrons travel from the negative to the positive electrode through an external circuit to power the load and complete the discharge reaction at the same time as ions flow directly from the negative side to the positive side through an intervening electrolyte or electrolytes. In rechargeable batteries (also referred to as secondary batteries), the electrons and ions travel in the opposite direction during recharge. Given the existence of established electrical grid systems, the use of batteries (e.g., rechargeable batteries) in the automobile industry could be an attractive option with regard to replacing the combustion engine, assuming that costs, safety, energy density and/or recharging times of these devices can be improved.
Lithium-air batteries are currently the subject of much scientific investigation due to their high theoretical energy density (i.e., of about 12 kWh/kg). When discharging, lithium metal anodes release lithium ions and an electron. The electron goes through an external circuit and lithium ions travel through electrolyte(s) to the cathode. Oxygen combines with the electrons and lithium ions to complete the reaction and produce lithium oxide or lithium peroxide (i.e., Li2O and Li2O2). When recharging, lithium oxide or lithium peroxide decomposes to produce oxygen and lithium ions travel back to the anode and are reduced to lithium metal. The oxygen electrode can be porous, e.g., to store the solid products generated from the reaction of Li ions with O2 (i.e., Li2O and Li2O2) during the discharge cycle of the battery and typically include a catalyst to promote reactions. Depending upon the type of electrolyte(s), four different types of lithium-air battery have been proposed: aprotic, aqueous, solid state, and mixed aqueous/aprotic. One issue, particularly with aprotic lithium-air batteries is that the overpotential to drive the product back to oxygen and lithium metal is large, usually over 1.5V.
Accordingly, there is an ongoing need for improved electrochemical systems, such as improved metal-air batteries, and for robust bifunctional catalysts (i.e., catalysts that can reduce both the charge overpotential and the discharge overpotential) that can be used in such systems, e.g., to improve efficiency and recharge rates. For instance, e.g., to help increase system efficiency, there is a need for reversible catalysts that can promote both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER).
Hydrogen peroxide is an important commodity chemical. Hydrogen peroxide can be used as a bleaching agent (e.g., in the paper industry) and as a cleaning and disinfecting agent. Hydrogen peroxide has also found use in the cosmetic industry and as a propellant. Most hydrogen peroxide is currently produced via an anthraquinone oxidation process, which results in significant chemical waste. Further, since the anthraquinone oxidation process is difficult to carry out on a small scale, production of hydrogen peroxide generally requires transport and storage, which can result in safety concerns. Hydrogen peroxide can also be produced on a smaller scale via direct synthesis from hydrogen and oxygen or electrochemically. These methods would reduce the need for storage and transport by allowing hydrogen peroxide to be produced on site, as needed. The direct synthesis method can involve its own safety concerns (e.g., due to flammability of mixtures of hydrogen and oxygen gases). Recent efforts have been made to improve ORR catalysts for the electrochemical synthesis of hydrogen peroxide; but there is still a need for additional, higher performance ORR catalysts for producing hydrogen peroxide.